1) Field of the Invention
This invention relates to a method for treating organic waste water by the methane fermentation, which method is adapted for treating organic waste water, such as waste liquor formed in the production step of beer or the like, through a biological treatment by the use of mesophilic or thermophilic methane bacteria, and an apparatus to be used for method.
2) Related Art Statement
As the above described treating method of organic waste water, there are known an aerobic treatment using of active sludge and an anaerobic treatment by the use of methane bacteria. Recently, there has been noticed a treating method, wherein an organic substance is decomposed into methane gas and carbon dioxide with the use of mesophilic or thermophilic methane bacteria by means of an anaerobic fixed bed. However, in the case where an alcohol waste liquor, such as waste liquor formed in the production step of beer, is treated by the methane fermentation, when the BOD load is as low as about 1-3) kg/m.sup.3 .multidot.day, the treatment can be effected without trouble, but when the BOD load is higher than about 3 kg/m.sup.3 .multidot.day, the BOD removal ratio is gradually lowered, and a satisfactorily high treating effect can not be obtained.
Japanese Patent Laid-open Application No. 64-47,498 discloses a method for improving the efficiency of methane fermentation by adding a mineral and a nutrient, such as amino acid or the like, to alcohol waste liquor. However, the inventors have tested this method in a practical operation and have concluded that this method is still unsatisfactory in the BOD load and BOD removal.
According to the investigation by the inventors, in the treating apparatus using an anaerobic fixed bed of this kind, its treating ability is highly influenced by the value of the ratio of the apparent surface area of the carrier, which carries a microorganism thereon, to the effective volume of the anaerobic treating apparatus. That is, in the anaerobic treatment, the BOD contained in waste water is decomposed into organic acid (through acid fermentation) by the action of acid fermentation bacteria contained in the liquid phase, and the resulting organic acid is decomposed into methane gas by the action of the methane bacteria fixed to the surface of a carrier. Therefore, it is important the methane fermentation in the second half stage proceeds in a rate higher than the rate of the acid fermentation in the first half stage in order for the anaerobic treatment to proceed smoothly. For this purpose, it is necessary that the value of the ratio of the surface area of the carrier, which is the residence of methane bacteria, to the volume of the liquid phase, which is the residence of acid fermentation bacteria, is not lower than a certain value, and it has been necessary that the value of the ratio of (surface area of carrier)/(effective volume of liquid) is at least 20 m.sup.2 /m.sup.3 as disclosed in Japanese Patent Laid-open Application No. 63-248,497.